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Subject: Re: [boost] [atomic] (op)_and_test naming
From: Peter Dimov (lists_at_[hidden])
Date: 2018-01-26 13:32:46

Andrey Semashev wrote:
> > While we're on the subject, on what architectures would opaque_sub be
> > more efficient than sub_and_test?
> On x86 and gcc < 7 opaque_sub allows to use "lock sub" or "lock dec"
> without setting the bool according to the zero flag, i.e. it saves a
> register and an instruction.

Right, thanks. I was thinking that testing for zero comes for free, but it's
not (entirely) free for the reason you give. Does this actually matter in
practice? I would expect the atomic to dominate the `set(n)z al`.

> Gcc 7 introduced the ability to return flags from the asm statement, so
> the code can be written the same way. Although I noticed that the compiler
> tends to save the flag into a register early unless it is tested
> immediately, so in some cases opaque_sub might still be preferable where
> it suits.

Don't see how opaque_sub could be preferable if you need to test the flag
later. :-) Presumably, if you just call the function and discard the return
value - the equivalent of opaque_ - the compiler would be smart enough to
not save the flag.

I remember some compilers being smart enough to notice that you don't use
the result of the atomic fetch_op intrinsic and generating the `lock op`
themselves, without a separate opaque_op being needed. We can't do that on
the library level, of course.

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